Hexavalent chromium compounds have been used in traditional conversion coatings to treat metal surfaces to improve their corrosion resistance and paint adhesion. Hexavalent chromium shows toxicological effects and has been determined by the Environmental Protection Agency as a risk to the environment and by the Occupational Safety and Health Agency as a health risk. Moreover, chemistries based on hexavalent chromium are classified as carcinogenic by these agencies.
Within the past few decades, various compositions and processes, not relying on hexavalent chromium, have been described and used for treating metal surfaces. One such example is described in U.S. Pat. No. 7,029,541 to Diaddario, Jr. et al., which describes a composition comprising chromium (III) ions.